The present invention relates to a method for imaging a printing form. The present invention is also directed to a device for imaging a printing form.
When imaging printing plates capable of being imaged once or multiple times, printing sleeves, printing belts, or printing cylinder surfaces (in this patent application generally referred to as “printing form” hereinafter), the image data for the print job is processed by a raster image processor (RIP), and usually provided to a laser imaging device (mostly using an infrared laser), which transfers or writes the data as image information to the surface or into an upper layer of the printing form in the form of a pattern.
For this purpose, the prior art has disclosed offline imaging devices (such as plate setters) using the internal drum, external drum, or flatted principles, which transfer the image information to the printing form to be produced, i.e., to be imaged, using the computer-to-plate process (CAP), and are therefore suitable for making printing forms. Such devices are described extensively, for example, in the “Handbook of Print Media”, Helmet Kipphan, Springer Verlag, Berlin, 2000 (hereinafter: Kipphan) on pages 597 through 626.
Also known from the prior art are inline imaging devices, which are used in direct imaging printing presses (DI presses), for example, in the Quickmaster 46-DI or the Speedmaster 52-DI of the Heidelberger Druckmaschinen company. In these devices, too, a laser imaging device is driven by a RIP and supplied with the data containing the image information in order to write the image information to the printing form, using the computer-to-press method. Devices of this kind are also extensively described in Kipphan, for example, on pages 627 through 656.
For laser imaging of printing forms, output powers of more than 1 watt per laser beam combined with highest beam quality may be required, depending on the type of plate, because the usually high imaging speed allows the beam to act on the imaging spots of the printing form only for a few microseconds, which is why energy for interaction with the printing form and for patterning the printing form at the respective location of the imaging spot can be deposited by the beam only during a rather short period of time.
For this reason, the lasers usually used for laser imaging are gas lasers, such as argon-ion lasers or helium-neon lasers, which, however, occupy a rather large space. Also used are solid-state lasers, such as Nd-YAG lasers, which require less space. Having an adequate power rating, all these lasers are capable of providing the energy required for imaging without amplification of the laser energy produced. The lasers are controlled and modulated in accordance with the image data.
Also known from the prior art are less expensive lasers requiring much less space, such as diode lasers which, in addition, have a longer average life, but are mostly limited to a power range below 1 watt. The use of such lasers to image printing forms would make it necessary to provide amplification.
Amplification of the power of diode lasers can be achieved, for example, using pumped fiber amplifiers.
For example, in the long-distance telecommunications environment, it is already known from German Patent Application DE 196 19 983 A1 to amplify the signal of a laser diode by means of an amplifier stage composed of erbium-doped standard single mode optical fibers and a pump light source in the form of a further laser diode. Such systems are referred to as MOPA (Master Oscillator Power Amplifier). The master oscillator—in this case the above-mentioned laser diode—has low laser power and highest beam quality.
However, it is a known characteristic of such fiber amplifier systems, which are cw-pumped (i.e., continuously supplied with energy), that they can emit a pulse caused by self-excitation; i.e., without external excitation by the diode laser signal to be amplified. Such a pulse will hereinafter be generally referred to as “interference pulse”. Since the fiber is pumped and, thus, supplied with energy continuously, the population inversion of the atoms or molecules involved in the amplification process can reach a level high enough for individual, spontaneously emitted photons to trigger a photon avalanche, and thus, to at least partially discharge the amplifier, thereby generating a pulse (this effect is called “self-q-switching effect”, and the pulse so generated will hereinafter be referred to as “self-q-switched pulse”).
Therefore, such an amplifier system cannot be used so easily for imaging printing forms because here, depending on the image information, for example, in the case of extensive non-printing areas which extend, in particular, in the circumferential direction, no imaging spot is to be produced during certain periods of time, and therefore, the fiber amplifier is not discharged by a signal of the imaging laser. Given a sufficiently long period of time, a self-q-switching effect can occur, as mentioned above, so that the fiber emits a signal independently, i.e., by self-excitation, which may lead to unwanted imaging in the form of an imaging spot, or destroy the output facet of the fiber.
Finally, from Japanese Patent Document JP 2001-27 00 70, where, for the purpose of imaging, a printing form is clamped to a cylinder, it is known to provide the image data for producing the printing form with so-called “dummy data”. This dummy data is inserted into the image data sequence at the locations that correspond to an angular position of the cylinder in which not the printing form but the cylinder gap for clamping the printing form comes to lie in the optical path of the imaging laser. Thus, the dummy data, which basically corresponds to empty image information, prevents the laser beam from entering the cylinder gap, and from being reflected there in an uncontrolled manner.